The invention relates to a method for lightening or eradicating pigments in human skin, in particular in pigmented skin lesions or tattoos.
Extremely short pulse (ESP), nanosecond or picosecond laser systems can successfully lighten or eradicate a variety of pigmented lesions. Pigmented lesions that are treatable include freckles and birthmarks including some congenital melanocytic naevi, blue naevi, naevi of Ota/Ito and Becker naevi. The ESP laser systems can also selectively destroy tattoo pigment without causing much damage to the surrounding skin. The altered pigment is then removed from the skin by scavenging white blood cells and tissue macrophages.
One of the techniques by which a laser can be made to produce a pulsed output beam is Q-switching (QS), sometimes known as giant pulse formation. The technique allows the production of light pulses with extremely short (in the range of nanoseconds) pulse duration and high (megawatt) peak power, much higher than can be produced by the same laser operating in continuous wave mode (constant output), or free-running pulse mode (0.1 ms-300 ms). There are also techniques by which laser pulses can be made to the even shorter (in the range of picoseconds). The ESP laser systems are effective because they confine their energy to the treated pigments. The time duration (pulse duration) of the ESP laser energy is so short that the extremely small pigments of a size of 10 nm-100 nm are heated to fragmentation temperature before their heat can dissipate to the surrounding skin. This prevents heating of the surrounding tissue that could potentially lead to burning or scarring of the skin.
The most likely cause of pigment destruction when subjecting the pigments to ESP laser pulses are shockwave and/or cavitation damage, the photomechanical physical effects produced from thermal expansion, and/or the extreme temperature gradients created within the melanosome or tattoo pigment. Melanin absorbs and localizes the high-intensity radiation from ESP lasers, thereby creating a sharp temperature gradient between the melanosome and surrounding structures. This gradient leads to thermal expansion and the generation and propagation of acoustic waves, which mechanically damage the melanosome-laden cells. For the selective removal of pigment, the color of the laser light must penetrate far enough into the skin to reach the target pigment and must be highly absorbed by the pigment relative to the surrounding skin. Different pigments therefore require different laser colors. For example, red light is highly absorbed by green tattoo pigments. In current practice, numerous lasers can specifically target pigmented lesions such lasers including red light lasers (e.g., 694 nm ruby, 755 nm alexandrite), green light lasers (e.g., 532 nm frequency-doubled Nd:YAG), and near-infrared lasers (e.g., 1,064 nm Nd:YAG).
Superficially located pigment is best treated with shorter wavelength lasers whilst removal of deeper pigment requires longer wavelength lasers that penetrate to greater tissue depths. For example, green light lasers (KTP lasers; KTP=potassium titanyl phosphate KTiOPO4) do not penetrate as deeply into the skin as the red light lasers and near-infrared lasers because of their shorter wavelengths. Therefore, green light lasers are effective only in the treatment of epidermal pigmented lesions. Caution should be used when treating darker-skinned people as permanent hypo-pigmentation and depigmentation may occur.
Q-switched Nd:YAG lasers produce a 1,064 nm wavelength beam with a pulse duration of typically 1 nanoseconds-25 nanoseconds. In comparison, pulse durations of QS ruby lasers and alexandrite lasers are typically longer, with durations up to 100 nanoseconds. Although the Nd:YAG wavelength is not absorbed as well by melanin as green light and red light wavelengths, its advantage lies in its ability to penetrate more deeply into the skin (up to 4 mm to 6 mm). A laser which produces 1,064 nm wavelength light is also more useful in the treatment of lesions for individuals with darker skin tones. In addition, the infrared wavelength light produced by a Q-switched Nd:YAG laser system can be converted into visible wavelength light. The latest devices incorporate an Nd:YAG (1,064 nm) laser as the main laser source from which all other wavelengths are created. The first wavelength converter is KTP crystal which has the ability to double the frequency of the incoming Nd:YAG beam and thus produce the halved wavelength of 532 nm (green light). For further wavelength conversions to 585 nm (yellow) and 650 nm (red), the KTP 532 nm laser beam is used as a source for optical pumping of solid-state dye lasers. Recently, solid-state dyes, capable of generating wavelengths other than 585 nm and 650 have also become available.
Picosecond ESP lasers with various wavelengths are also available. An example is an alexandrite laser with a pulse duration of 500 picoseconds to 900 picoseconds which has been used for tattoo removal.
In addition to the appropriate choice of laser color in order to be best absorbed by the color of the pigment, a laser system must be able to deliver extremely short pulses with very high pulse energy. The fluence (F) is one of the main parameters for treating pigments. It is defined as energy density: F=E/A, where E is the energy of the laser pulse and A is the spot size area of the laser beam at the skin surface. Sufficient laser fluence must be delivered during each laser pulse to heat the pigment to cause fragmentation. If the fluence is too low, the pigment will not fragment and no removal will take place.
Pigment removal is based on a process of pigment disintegration caused by strong acoustic waves generated during the interaction between ESP laser pulses and the pigment particles. The pigment particles are then more easily removed by the body's own immune system. Several treatment sessions are typically required with intervals of three weeks between sessions to allow the pigment residue to be cleared by the body. It is therefore highly desirable for patients and practitioners to maximize the efficacy of each treatment session and to reduce the number of sessions needed.
One way of possibly increasing treatment efficacy is to increase the energy of the produced acoustic waves by increasing the laser fluence. However, with larger energies of acoustic waves, side effects start to appear more frequently. Cavitation bubbles, which are formed around the pigment particles due to their increased temperature, and plasma formation can damage the surrounding tissue. Measurements of acoustic waves clearly show the existence of a laser fluence threshold at which uncontrolled skin perforation and subsequent scarring as a side effect occur. This threshold represents the limit for the applied laser pulse parameters. Occasional side effects such as depigmentation, allergic reactions, ink darkening, and epidermal debris have been reported as well.
Another option for increasing treatment efficacy is to deliver more than one ESP laser pulse to the treated area during the same treatment session. However, applying a sequence of laser pulses is not as effective as using a single large pulse. Since tissue characteristics change following the irradiation with a laser pulse, the pigment removal efficacy is reduced for subsequent laser pulses. During an ESP pulse, temperatures can exceed 1,000° C. The gaseous products of pyrolysis and superheated steam account for the immediate whitening of the treated skin. Whitening results in an optical shield that prevents subsequent laser pulses from reaching the remaining underlying pigments.
There is therefore the need for an improved method of pigment removal which permits the use of higher laser intensities while reducing the probability of side effects.